Annealing and TMOS coating on PSF/ZTC mixed matrix membrane for enhanced CO2/CH4 and H2/CH4 separation

Recently, natural gas (mostly methane) is frequently used as fuel, while hydrogen is a promising renewable energy source. However, each gas produced contains impurity gases. As a result, membrane separation is required. The mixed matrix membrane (MMM) is a promising membrane. The huge surface area and well-defined pore structure of zeolite templated carbon (ZTC)-based MMM allow for effective separation. However, the interfacial vacuum in MMM is difficult to avoid, contributing to poor separation performance. This research tries to improve separation performance by altering membrane surfaces. MMM PSF/ZTC was modified by annealing at 120, 150, and 190°C; coating using 0.01, 0.03, and 0.05 mol tetramethyl orthosilicate (TMOS); and a combination of both, i.e. annealing at 190°C and coating using 0.03 mol TMOS. MMM PSF/ZTC successfully significantly improved CO2/CH4 selectivity by a combination of annealing at 190°C and coating 0.03 mol TMOS from 1.37 to 5.90 (331%), and H2/CH4 selectivity by coating with 0.03 mol TMOS from 4.58 to 65.76 (1378%). The enhancement of selectivity was due to structural changes to the membrane that became denser and smoother, which SEM and AFM observed. In this study, annealing and coating treatments are the methods investigated for improving the polymer matrix and filler particle adhesion.

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Dear Dr Widiastuti: Title: Selectivity Improvement of Polysulfone-Zeolite Templated Carbon Membrane by Annealing and Coating Treatment for CO2/CH4 and H2/CH4 Separation Manuscript ID: RSOS-211371 Thank you for your submission to Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) this work reported that the development of ZTC MMMs for CO2/CH4 and H2/CH4 separations, the topic is very important for industrial application when it is applied to natural gas sweetening, and H2 recovery from the natural gas grid. however, the prepared membranes do not provide a significant improvement compared to the state-of-the-art membranes reported in the literature or industrial applications. therefore, I feel this work did not provide significant scientific novelty for publishing in RSOS.
(1) based on the SEM images in Figure 4. it seems the ZTC particles caused a defect on the surface of the membranes. due to the large finger-like structure of the PSF MMMs, it is unlike the ZTC will provide any contribution neither to gas permeance nor selectivity. by post-treatment with annealing or coating TMOS selective to cause cross-link or make an extra selective layer to enhance the selective might provide some novelties, however, the significant reduction of gas permeance makes it unsuccessful for the post-treatment. as we can see from Figs. 7 and 8, those post-treated membranes do not present a better performance compared to the fresh membrane with respect to the Robeson upper bound. the slightly increased selectivity may not offset the significantly reduced permeance.
(2) the authors need to add the pure PSF membranes as the reference to document the effect of adding ZTC nanoparticles (3) it is unclear why the membrane coated with 0.03 TMOS presented a much higher H2/CH4 selectivity compared to other membranes (&lt;10). this data point needs to be checked or at least explained. also, the CO2/CH4 selectivity is less than 1-unlikely (it might be the experimental error that there is no selectivity), error bar should be given. (4) based on the SEM images of Fig. 1H and K, it is clear that the TMOS penetrates deeply into the matrix, which leads to the significant reduction of gas permeance, and of course a slight increase of the selectivity. The reviewer believes that the pore size of fresh PSF/ZTC MMMs is too large for direct coating.
Reviewer: 2 Comments to the Author(s) The manuscript is more detailed, with clear and crisp tables of photographs. The materials and experimental methods are more complete and will facilitate research by other scholars. I consider it to be a manuscript that could be published.
Reviewer: 3 Comments to the Author(s) 1. In Fig.1, the dense structure formed by the annealing treatment looks like a new layer formed on the PSF matrix, and H2 CO2 CH4 were not marked out. 2. The manuscript lacks the characterization of ZTC itself, including but not limited to the pore size. 3. Whether it is possible to accurately characterize the pore size change range of the membrane before and after annealing. 4. The performance of binary gas H2/CH4 was only carried out using MMM annealed at 190℃, how about other types of membranes? 5. Please illustrate the reason of CH4 increased permeance and CO2 /CH4 increased selectivity occurred on MMM coating with 0.05 mol TMOS. 6. "the excess TMOS concentration on the membrane was able to form multilayers, which could reduce adhesion between the polymer matrix and filler particles." The analysis is confused.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation? Accept as is
Comments to the Author(s) 1) the title might be a bit confusing as the reader might confuse with carbon membranes (in fact this is a PSF/ZTC MMM) 2) the title indicated the enhancement for CO2/CH4 separation by annealing and coating treatment. however, from Table 5, the reviewer cannot clearly see the improvement at least for CO2/CH4 separation, for some cases, it decreases. and the authors may not fully address my comment why CO2/CH4 selectivity is lower than 1. what is the transport mechanism.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Accept as is

Comments to the Author(s)
The author is familiar with is manuscript content and revises it carefully and completely with a serious attitude. The structure of the article is clear. Recommended for acceptance.
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Dear Dr Widiastuti: Title: Selectivity Improvement of Polysulfone-Zeolite Templated Carbon Membrane by Annealing and Coating Treatment for CO2/CH4 and H2/CH4 Separation Manuscript ID: RSOS-211371.R1 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Supplementary files will be published alongside the paper on the journal website and posted on the online figshare repository (https://figshare.com). The heading and legend provided for each supplementary file during the submission process will be used to create the figshare page, so please ensure these are accurate and informative so that your files can be found in searches. Files on figshare will be made available approximately one week before the accompanying article so that the supplementary material can be attributed a unique DOI. ************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) 1) the title might be a bit confusing as the reader might confuse with carbon membranes (in fact this is a PSF/ZTC MMM) 2) the title indicated the enhancement for CO2/CH4 separation by annealing and coating treatment. however, from Table 5, the reviewer cannot clearly see the improvement at least for CO2/CH4 separation, for some cases, it decreases. and the authors may not fully address my comment why CO2/CH4 selectivity is lower than 1. what is the transport mechanism.
Reviewer: 2 Comments to the Author(s) The author is familiar with is manuscript content and revises it carefully and completely with a serious attitude. The structure of the article is clear. Recommended for acceptance.

Recommendation? Accept as is
Comments to the Author(s) I'm satisfied with the revision, and recommend to publishing this work.
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Dear Dr Widiastuti: Title: Annealing and TMOS coating on PSF/ZTC mixed matrix membrane for enhanced CO2/CH4 and H2/CH4 separation Manuscript ID: RSOS-211371.R2 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
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RE: Point-by-point response for revision to manuscript RSOS-211371
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Response to Reviewers of Royal Society Open Science
Reviewer #1: this work reported that the development of ZTC MMMs for CO2/CH4 and H2/CH4 separations, the topic is very important for industrial application when it is applied to natural gas sweetening, and H2 recovery from the natural gas grid. however, the prepared membranes do not provide a significant improvement compared to the state-of-the-art membranes reported in the literature or industrial applications. therefore, I feel this work did not provide significant scientific novelty for publishing in RSOS. 1 based on the SEM images in Figure 4. it seems the ZTC particles caused a defect on the surface of the membranes. due to the large finger-like structure of the PSF MMMs, it is unlike the ZTC will provide any contribution neither to gas permeance nor selectivity. by post-treatment with annealing or coating TMOS selective to cause cross-link or make an extra selective layer to enhance the selective might provide some novelties, however, the significant reduction of gas permeance makes it unsuccessful for the posttreatment. as we can see from Figs. 7 and 8, those post-treated membranes do not present a better performance compared to the fresh membrane with respect to the Robeson upper bound. the slightly increased selectivity may not offset the significantly reduced permeance. Response First of all, thank you for your time to review this manuscript.
We greatly appreciate the reviewer's positive comments. Our response to your comments is below: There is no relationship between the defects and the finger-like structure. Based on Wijiyanti et al. [1], the neat membrane does have finger-like pores similar to this study. The finger-like pore was obtained as a result of phase inversion between coagulation liquid and polymer solution during the dry/wet-spinning process. Thus, it cannot be expressed as a defect. The defect that occurs in MMM is in the form of interfacial voids, which is shown in Figure b. However, in this study, there is no visible defect on the membrane, as shown in Figures 4b1, d1, f1.
Page 5 Line 36-39. According to the SEM observation, the finger-like pore was formed during the dry/wetspinning process as a consequence of phase inversion between the coagulation liquid and polymer solution. On the other hand, the presence of voids in MMM PSF/ZTC without annealing, as shown in Wijiyanti et al. [1], was due to the low adhesion between the polymer matrix and the ZTC. Indeed, the annealing and TMOS coating process does not provide a significant increment in gas separation performance. However, it should be noted that annealing is important in improving the mechanical properties of membranes by promoting the strengthened interactions among polymer chains, and a higher degree of crystallinity in the polymer matrix. Pham et al. [2]  On the other hand, the addition of TMOS did cause a decrease in permeability, which can be shown by SEM images which are much denser in structure (Figure 4g, h). This led to a significant increase in selectivity in the membrane with 0.03 mol TMOS for the studied gas separation. Furthermore, a significant increase in selectivity occurred in the gas separation process with different gas sizes, especially in the H2/CH4 gas separation. As a result, the addition of TMOS in an optimum composition could increase membrane performance. For instance, the membrane coated with 0.03 mol TMOS exhibits superior H2/CH4 separation performance. 2 the authors need to add the pure PSF membranes as the reference to document the effect of adding ZTC nanoparticles. Response We highly recognize the reviewer's positive comments. However, in this study, we focus on the investigation of post-treatment MMM PSF/ZTC to improve gas separation performance. We have been discussed the effect of adding ZTC fillers in the PSF membrane, as well as the neat PSF membrane characterization that has been exhibited in the previous studies [1,3].
Furthermore, we have discussed the addition ZTC nanoparticle could enhanced CO2/CH4 and H2/CH4 gasses separation performance of polysulfone membrane.
Page 2 Line 39-41. Our result showed that the presence of ZTC as filler in the MMM-based polysulfone increased the selectivity of CO2/CH4 from 2.56 to 9.99 and selectivity of H2/CH4 from 7.77 to 28.88 [1]. 3 it is unclear why the membrane coated with 0.03 TMOS presented a much higher H2/CH4 selectivity compared to other membranes (<10). this data point needs to be checked or at least explained. also, the CO2/CH4 selectivity is less than 1-unlikely (it might be the experimental error that there is no selectivity), error bar should be given.

Response
We highly recognize the reviewer's positive comments, we have added error bar in all Figures and standard deviation in Table 2.
Page 16. We suggest that 0.03 mol TMOS addition is provided an ideal coating for the membrane to separate H2/CH4. Furthermore, for CO2/CH4 gas separation, the addition of 0.05 TMOS gives the best results in terms of selectivity, but if we look at the trend, it is actually still necessary to carry out further variations on the higher addition of TMOS concentration (which will be considered in future research). According to the H2/CH4 separation performance, the addition of TMOS is optimal at 0.03 mol. We guess the increase in concentration is the optimal condition to produce a membrane with the appropriate structure in the H2/CH4 separation process. It is also exhibit similar result from Kagari et al. [4], in which they study coating PEI membrane utilizes PDMS at various concentrations (5-17 wt%). The selectivity of H2/CH4 was increased utilizing coating solutions up to 15.0 wt%, but thereafter decreased as the concentration increased. Further increasing the coating solution concentration seems to have resulted in a thicker layer with low gas permeance. On the other hand, the selection size of the silane coating agent influences the gas diffusion compatibility. For example, PVP was not suited for surface modification of microporous inorganic fillers since polymer sizing on the particle surface is prone to causing pore blockage [5]. This argument is used for 0.03 mol TMOS utilization, which could be disturbing the diffusion of higher size of gas molecules (i.e., CO2 and CH4) due to the membrane pores are getting narrower, while H2 penetration not affected considerably.
We have added discussion. Page 3 Line 9-13. On the other hand, the selection size of the silane coating agent influences the gas diffusion compatibility. For example, PVP was not suited for surface modification of microporous inorganic fillers since polymer sizing on the particle surface is prone to causing pore blockage [5]. Thus, this study discovered the other potential coating material which has unique properties to enhance gas separation performance.
Page 8 Line 34-36. It suggests that at 0.03 mol TMOS utilization could be disturbing the diffusion of higher size of gas molecules (i.e., CO2 and CH4) due to the membrane pores are getting narrower, while H2 penetration not affected considerably. As seen in Figure 7 (a-c), the MMM pore was smaller.
4 based on the SEM images of Fig. 1H and K, it is clear that the TMOS penetrates deeply into the matrix, which leads to the significant reduction of gas permeance, and of course a slight increase of the selectivity. The reviewer believes that the pore size of fresh PSF/ZTC MMMs is too large for direct coating. Response We are aware of it and we also would like to suggest for coating should only be dyed. We do it under reflux expecting a cross link between the TMOS and the membrane. However, we did not find the cross-linking behavior according to the FTIR spectra.
Reviewer #2: The manuscript is more detailed, with clear and crisp tables of photographs. The materials and experimental methods are more complete and will facilitate research by other scholars. I consider it to be a manuscript that could be published. Response First of all, thank you for your time to review this manuscript. Thank you for your positive feedback. We highly appreciate it.
Reviewer #3: 1 In Fig.1, the dense structure formed by the annealing treatment looks like a new layer formed on the PSF matrix, and H2 CO2 CH4 were not marked out. Response First of all, thank you for your time to review this manuscript.
The reviewer raises an interesting concern. Thank you very much for your positive comment, we have been redrawing the figure.
Sorry for making it confusing, but it's not. Please kindly see the revised illustration for a better understanding. The manuscript lacks the characterization of ZTC itself, including but not limited to the pore size. Response We highly recognize the reviewer's positive comments. However, we have been discussed more detailed ZTC characterization in previous research in [1,3,6]. 3 Whether it is possible to accurately characterize the pore size change range of the membrane before and after annealing. Response We highly recognize the reviewer's positive comments. Tsuru and coworkers have been reported the utilization of nanopermporometry apparatus to determine the membrane pore size [7][8][9][10][11]. However, we did not have access to conducting this characterization currently. We would consider studying the pore size change analyzation of the mixed matrix membrane in further research as it is also the important feature for aid elucidating the separation processes in membrane. 4 The performance of binary gas H2/CH4 was only carried out using MMM annealed at 190℃, how about other types of membranes? Response We highly recognize the reviewer's positive comments. We chose a membrane that has been annealed at 190 °C since it has the highest separation performance in single gas separation, and we presume that mixed gas separation is not much different. 5 Please illustrate the reason of CH4 increased permeance and CO2 /CH4 increased selectivity occurred on MMM coating with 0.05 mol TMOS. Response We apologize with the data drawn on the graph is upside down for CO2 and CH4 permeation (Figure 6a). Thus, we have redrawn it and rechecked all data carefully. Figure. 6 (a) Permeation and selectivity of (a) CO2/CH4 (Page 9 Line 1) As the concentration of the TMOS coating increases, it covers the membrane pores so that the most influential diffusion is solution diffusion, which is more influenced by the solubility of the gas. Therefore, the H2 gas permeation decreased while the CO2 and CH4 gas permeation increased. Furthermore, we suggest that the addition of silane which consist of oxygen atom promotes physical interaction due to higher polarity [12,13]. Thus, CO2 permeation higher than CH4 as well as the CO2/CH4 selectivity increase following by increasing TMOS concentration coating.
We have added this discussion below, Page 8 Line 44-50: However, at 0.05 mol TMOS coating it observed a unique pattern that the penetration of H2 gas reduced while the permeability of CO2 and CH4 gas climbed. Because this concentration of the TMOS coating rises, it completely covers the membrane pores, favoring solution diffusion, which is more dependent on the gas's solubility. Moreover, it suggests that the incorporation of silane, which contains oxygen atoms, facilitates physical contact owing to its increased polarity [12,13]. Thus, increasing the TMOS concentration coating leads to a rise in CO2 permeation greater than CH4 permeation, as a result, CO2/CH4 selectivity improved 6 "the excess TMOS concentration on the membrane was able to form multilayers, which could reduce adhesion between the polymer matrix and filler particles." The analysis is confused. Response We highly recognize the reviewer's positive comments. Thank you for your concern.
We have revised the sentences.
the excess TMOS concentration on the membrane was able to form multilayers, which can not only cover the voids but also block the gas diffusion path.

Conclusion 8
The logic of the manuscript does not match the order of figures placement and should be adjusted appropriately. Response We highly recognize the reviewer's positive comments, our response to your comments below: Thank you for the constructive suggestion, we have reorganized it.
From Figure 4.    Additionally, Figure 9 create from combining of SEM, and AFM characterization of annealing and coating combination post-treatment in membrane.
Page 10 Line 24. Lastly, the Robeson upper bound discussion move to Page Page 11 Line 28-32 and Page 12 Line 1-11. The comparison between the performance of CO2/CH4 and H2/CH4 gas separation, and the Robeson upper bound curve can be seen in Figure 11. MMM PSF/ZTC without annealing and MMM PSF/ZTC annealed at 190 °C had a gas separation performance of CO2/CH4 close to the Robeson upper bound curve and H2/CH4 gas separation performance, which was above the Robeson upper bound curve. Those exhibited a good separation performance, but the selectivity was still relatively low. On the other hand, The MMM PSF/ZTC without coating and MMM PSF/ZTC coated with 0.01 and 0.03 mol TMOS had H2/CH4 gas separation performance was above the Robeson upper limit, indicating good gas separation performance. Unlike the case with the CO2/CH4 gas separation performance, which was quite far from the Robeson upper limit. Furthermore, the gas separation performance of MMM PSF/ZTC modified with combinations of annealing and coating was no better than MMM PSF/ZTC modified by annealing or coating only. Therefore, the appropriate membrane modification to improve the performance of MMM PSF/ZTC was annealing at 190 °C or coating with 0.03 mol TMOS. Figure 11. MMM PSF/ZTC gas separation performance with variations in heating temperature, coating concentration, and its combination for gas pairs (a) CO2/CH4 and (b) H2/CH4, compared to the Robeson curve [15] and other studies [16][17][18].
The preparation and application of MMM has not been reviewed completely, such as 10 We highly recognize the reviewer's positive comments. However, the cited paper is not relevant with this study. In this study we focused on mixed matrix membrane PSF/ZTC for gas separation application. The study from (10.1016/j.cej.2020.127144; 10.1016/j.seppur.2019.05.009; 10.1166/jnn.2017.13914; 10.1016/j.memsci.2016.07.060) was focus on nanofiltration and ultrafiltration. Therefore, we regret that we could not cite it since the topics discussed are wide and do not focus on gas separation applications. Furthermore, we have been reviewed the previous study of MMM for gas separation.
Our result showed that the presence of ZTC as filler in the MMM-based polysulfone increased the selectivity of CO2/CH4 from 2.56 to 9.99 and selectivity of H2/CH4 from 7.77 to 28.88 [1]. The other fillers applied in MMM based polysulfone have been reported by several researchers such as zeolite and silica. Mohamat et al. [19] reported that the incorporation of 3wt% zeolite T in polysulfone membrane enhanced CO2/CH4 selectivity from 2.63 to 3.37 with CO2 permeability as 78.90 GPU. On the other hand, the presence 2wt% of KIT-6 (KIT: Korea Advanced Institute of Science and Technology), a silica mesoporous, could improve CO2/CH4 selectivity for 32.4 with CO2 permeability as 5.4 Barrer [20].
On the other hand, thought the preparation method of this study is utilizing phase inversion, it's quite different preparation technique in this study and the cited literature. In addition, we have been cited more similar preparation of hollow fiber MMM that adopted from previous studies.
Page 4 Line 36-37. The membrane preparation is adopted from the previous literatures [1,3]. 10 In Figure 4, the uppercase and lowercase letters are inconsistent with the legends, and there is no description for fig.4l. Response Thank you for your concern about it, we have revised it. Please kindly see revised